This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A020509 #15 Feb 26 2024 10:59:37
%S A020509 -10,-11,-9,91,101,9091,111,909091,10001,999001,11111,9090909091,9901,
%T A020509 909090909091,1111111,109889011,100000001,9090909090909091,1001001,
%U A020509 909090909090909091,99009901,1098900989011,11111111111
%N A020509 Cyclotomic polynomials at x=-10.
%H A020509 Paolo Xausa, <a href="/A020509/b020509.txt">Table of n, a(n) for n = 0..900</a>
%H A020509 <a href="/index/Cy#CyclotomicPolynomialsValuesAtX">Index entries for cyclotomic polynomials, values at X</a>
%p A020509 with(numtheory,cyclotomic); f := n->subs(x=-10,cyclotomic(n,x)); seq(f(i),i=0..64);
%t A020509 Join[{-10}, Cyclotomic[Range[30], -10]] (* _Paolo Xausa_, Feb 26 2024 *)
%K A020509 sign
%O A020509 0,1
%A A020509 _Simon Plouffe_